1. Field of the Invention
The present invention relates to a fuel cell system, and in particular, relates to a technique of controlling an electric current drawn from fuel cells.
2. Description of Related Art
A household fuel cell system (power generating unit) generally includes a hydrogen production device including a reformer which reforms a hydrocarbon fuel (e.g., municipal gas, LPG, or kerosene) to produce hydrogen; fuel cells (a fuel cell stack) which generates direct current power by an electrochemical reaction between the produced hydrogen and oxygen in air; a power conditioner which draws the direct current power generated by the fuel cells and converts the power to alternating current power to thereby supply the converted power to domestic appliances; and a heat exchanger which collects heat generated by the fuel cells or the like, and exchanges the heat with a hot-water supply unit side.
For such a fuel cell system, there is known a method in which a target power generation value is set in accordance with demand power, and based on the set value, a fuel supply and the like are adjusted to thereby control the power generation. On the other hand, based on the desired power generation value, an electric current drawn from fuel cells is controlled, and the electric current drawn from the fuel cells is compared with an upper limit of the electric current and then the electric current is limited.
Furthermore, in Japanese Laid-Open Patent Publication No. 2006-309979, if an output voltage of fuel cells is lower than a lower limit of a voltage at which the fuel cells can stably generate power, the electric current drawn from the fuel cells is limited.
In a conventional fuel cell system, an electric current drawn from fuel cells is controlled based on a target power generation value. However, if the target power generation value suddenly increases and then the electric current drawn from the fuel cells suddenly increases, an output voltage of the fuel cell suddenly decreases in accordance with I-V characteristics, and the output voltage is lower than a lower limit of a voltage at which the fuel cells can stably generate power, leading to the suspension of an operation occasionally.
Therefore, the electric current drawn from the fuel cells is limited by setting an upper limit of the electric current, but conventionally, the upper limit of the electric current is set in accordance with the target power generation value, and when the electric current is momentarily close to the upper limit of the electric current, a voltage drops in excess of the lower limit of the voltage, failing to prevent the suspension of the operation.
Furthermore, in Japanese Laid-Open Patent Publication No. 2006-309979, if the output voltage of the fuel cells is lower than the lower limit of the voltage at which the fuel cells can stably generate the power, the electric current drawn from the fuel cell is limited. However, this control is carried out after the occurrence of the voltage drop of the fuel cells, and hence there is a problem that the voltage drop is not able to be prevented in advance.
In view of such circumstances, it is an object of the present invention to optimally set an upper limit of an electric current drawn from fuel cells, thereby decreasing the suspension of the operation caused by voltage drop as less frequently as possible.